1. Field of the Invention
The present invention is related to apparatus for testing the hysteresis loop characteristics of magnetic layers by employing the longitudinal magneto-optic Kerr effect. More particularly the present invention relates to a novel apparatus for non-destructively testing thin magnetic layers which are deposited on rigid or flexible substrates of arbitrary dimensions such as magnetic tape, floppy disc and hard disc for computer use as well as magnetic thin film transducers such as inductive and magneto-resistive thin films for thin film head structures. The term "non-destructively testing" as employed in this specification shall mean the product is tested without the need for cutting out samples to be measured, thus, destroying the product as required in the prior art.
2. Description of the Prior Art
Apparatus for testing the hysteresis loop of magnetic layers is known. Such apparatus are commercially available which employ inductive techniques and are commercially known as vibrating sample magnetometers or inductive hysteresis loop tracers. Such devices are well known in the art and require a large field in order to test the sample which is placed in the gap of a large electromagnet thus requiring destruction of the product from which the sample was taken.
Another form of hysteresis loop testing apparatus is known as magneto-optic Kerr effect apparatus which also requires that the sample be placed in the gap of a large electromagnetic thus requiring that the product be destroyed to create the sample. This latter type testing apparatus has been reported in IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG.-22, No. 5, September 1986 at pages 662-664 entitled "Magneto-optic Kerr Effect Hysteresis Loop Measurements on Particulate Recording Media" by Dr. Richard M. Josephs, Dr. Charles S. Krafft and Douglas S. Crompton. In this reference I described the gap length as being 7.6 centimeters, which restricts the size of sample cut from a product to less than half this dimension, thus, this apparatus was incapable of testing non-destructively large products such as magnetic recording disc in the gap space or outside the deep gap.
Advances in magnetic recording technology are leading to thinner and thinner magnetic films which are currently approaching 500 angstroms (.ANG.). Such thin magnetic films can be tested in the prior art testing apparatus mentioned above provided that samples are taken from the finished product. Quality control requires that tests be made of such critical thickness thin films on the production line during manufacture to avoid production of unacceptable product. The prior art testing techniques require extensive time to prepare the samples to be tested in the prior art devices which would permit the production line to create large amounts of scrap product before being discovered.
It would be extremely desirable to provide an economical, predictable and repeatable nondestructive hysteresis loop testing apparatus regardless of the principal employed. No such prior art testing apparatus is known or available today.